1. Field of the Invention
The present invention relates to optical fiber cables and, more particularly, to a polypropylene-polyethylene copolymer core or buffer tube for an optical fiber cable, and a method for making the same.
2. Description of the Prior Art
Optical fiber cables have been used for many years to transmit information at high rates and very long distances. The transmission medium of the optical fiber cable are hair-thin optical fibers protected from external forces and elements by precisely designed and manufactured cable structures. There are several relatively common cable structure families currently being used to protect these hair-thin optical fibers. Such cable structure families include the loose tube, the slotted core and the monotube structure.
In an optical fiber cable of the loose tube type, the optical fibers reside in a plurality of gel filled buffer tubes. These buffer tubes are stranded around a central strength member. In a slotted core type optical fiber cable, the optical fibers reside in gel filled channels or slots. These channels are symmetrical and form a helical path along the longitudinal axis of the cable. In an optical fiber cable of the monotube structure type, the optical fibers reside in a single, centrally located, gel filled buffer tube. In each of the family of structures, the buffer or core tube provides the primary structure to protect the hair-thin optical fibers residing therein.
In each type of optical fiber cable structure, it is desirable to have a buffer or core tube made from a material with a high Young's modulus. The use of a material with a high Young's modulus results in a cable with a buffer or core tube having relatively high tensile and compressive resistance capability, a trait useful in protecting the optical fibers in the event the cable is twisted, stretched or compressed. Also, it is important to select a material for the buffer or core tube that has a low thermal expansion coefficient. Too much shrinkage or expansion of the core or buffer tube caused by temperature changes could place tensile or compressive loads on the optical fibers residing therein. High tensile or compressive loads can result in damaged or broken optical fibers.
In each of the optical fiber cable structures discussed above, the prior art buffer or core tubes have been made from polybutylene terephthalate (PBT), polycarbonate (PC), a layered combination of PBT and PC, or a polyamide such as Nylon-12. The PBT, PC or Nylon are fairly good materials for making buffer or core tubes because they are materials that have high Young's modulus and low thermal expansion coefficients. However, such materials also have some disadvantages when compared to other materials such as polypropylene-polyethylene copolymers. These disadvantages include higher cost, lower flexibility, moisture sensitivity, and increased difficulty in handling and processing due to the mechanical properties of the materials.
While polypropylene-polyethylene copolymers are cheaper and easier to handle than PBT, PC or Nylon and could be used as buffer or core tubes for optical fiber cables, they generally have not been favored over PBT, PC or Nylon. The polypropylene-polyethylene copolymers generally have higher thermal expansion coefficients and lower Young's modulus than PBT, PC or Nylon such that a buffer or core tube made with the polypropylene-polyethylene copolymer would have greater shrinkage, and less compression-tension resistance. Prior to the present invention, the disadvantages of greater shrinkage and lower compression-tension resistance of polypropylene-polyethylene copolymers have outweighed the material handling and cost advantages offered by this material.